Conventionally, some servo system has a function of saving information in a servo computation as history data in a memory serving as a storage unit and reading out history data to an external apparatus connected via a host apparatus.
For example, as shown in FIG. 7, in each servo amplifier 71 of conventional servo system 70 that controls a plurality of servo shafts, information in a servo computation in servo computation unit 74 is saved as history data Sv in memory 76. Saved history data Sv is read out to external apparatus 95 via host apparatus 85 and used for, for example, adjustment of servo control gain. Saved history data Sv is information corresponding to past m (where m is an integer) servo computation periods. Examples of history data Sv include a current motor position and a current motor speed.
Referring to FIG. 7, host apparatus 85 and the plurality of servo amplifiers 71 provided in servo system 70 are connected to each other via communication line 81. Assume that when servo computation units 74 of the plurality of servo amplifiers 71 execute computations, time axes of servo computation information are aligned. In this case, servo amplifier 71 can adjust the respective servo shafts more accurately.
For example, Patent Literature 1 discloses a technique configured to establish synchronization between a communication timing and servo computation processing.
In addition, as a technique for aligning the time axes of history data Sv based on the respective servo computations, there is disclosed an arrangement configured to cause host apparatus 85 to instruct servo amplifiers 71 to start saving or stop saving history data Sv in the arrangement shown in FIG. 7 described above (see, for example, Patent Literature 2).
That is, in conventional servo system 70 described above, host apparatus 85 needs to issue an instruction to stop saving history data Sv.
In recent years, open networks including EtherCAT (registered trademark) have been popularized. Along with the popularization of open networks, therefore, an increasing number of servo systems include combinations of different providers for host apparatuses and servo amplifiers. Accordingly, when a host apparatus has no function of issuing an instruction to stop saving servo computation information, the time axes of the respective pieces of servo computation information cannot be aligned.